The present invention relates to a method and apparatus for non-invasively measuring certain physiological parameters of a subject. The invention is particularly useful in venous occlusion plethysmography (VOP) for measuring volumetric blood flow, or other physiological parameters derived from volumetric blood flow, as well as providing certain-quantitative indices of venous compliance, and is therefore described below with respect to such applications.
Venous occlusion plethysmography is a known method for non-invasively measuring a physiological parameter of a subject, particularly volumetric blood flow. It is based on the accumulation of blood in the veins of a body region that is distal or upstream from a venous occlusion device. In the venous occlusion plethysmography method, a pressure greater than the venous pressure, but less than the arterial pressure, is applied around a body region in order to cause venous blood to be accumulated in another body region located distally (or further away from the heart) to the one acted upon by the venous occlusion device. Changes in volume with respect to time of the distal region are measured and utilized to provide a measure of the blood flow, or other physiological parameters derived therefrom such as the relative volume accumulated at a given occlusion pressure level.
VOP is based on the following physiological properties of a subject's circulatory system:
1. the veins possess very high levels of compliance and are able to accommodate large amounts of blood at relatively low pressure;
2. there is a large pressure difference between systemic arterial and systemic venous blood pressure; and
3. blood flow in the systemic arteries propagates from the heart toward the periphery of the body, whereas it propagates in the opposite direction in the system veins, (i.e. from the periphery towards the heart).
At the present time, VOP measurements are generally made by inflating a segmental cuff, placed over the perimeter of a body region, to apply pressure over the entire circumference of the particular body region, to a pressure between venous and arterial pressure, and then measuring the volume of blood which accumulates distal to the cuff. As described more particularly below, when the venous occlusion pressure is intermittently raised in the cuff, the cumulative volume begins to increase with each pulse wave, until such time as the veins are unable to accommodate more volume.
As mentioned, the veins are characterized as having a very high degree of compliance compared to arterial blood vessels; that is, they are capable of undergoing a larger volume change for a given increase in pressure compared to equivalent sized arterial vessels. Compliance of the veins is especially high after they have initially been emptied. However as the veins become increasingly filled, and the venous wall begins to be stretched, the ability of the veins to expand without increasing the pressure of the contained blood tends to be reduced, upon pressure increases, thereby resulting in a non-linear pressure versus volume relationship. Nevertheless, a substantial portion of the volume of blood which the veins are able to accommodate from their fully emptied state, occurs without the venous wall being stretched, and this occurs at a very low level of pressure. The rate of change of the accumulated blood volume is the volumetric blood flow (BF). When the blood in the region distal to the occlusion cuff reaches the cuff pressure, further accumulation of blood ceases, and a steady state level of blood volume prevails. The volume of accumulated blood distal to the occlusion cuff at that point thus reaches a plateau which represents the venous capacitance (VC).
It is to be particularly noted that as the volume and pressure of the blood distal to the venous occlusion cuff increases, the compliance of the veins tends to decrease.
A major limitation of VOP, as it is currently practiced, is that it is restricted to measurements in body parts which are maintained above heart level, for the following reason: The combination of extremely high venous compliance described above, and the generation of hydrostatic gradients in the vascular system due to vertical displacement relative to heart level, means that the veins in body parts which are below heart level easily become filled with blood and therefore lose the capacity to freely accommodate additional volumes of blood. When venous blood vessels become filled due to hydrostatic pressure gradients, the partially filled veins cannot be accurately used for VOP measurements due to the relative reduction in the ability to further expand and the lack of linearity of the pressure volume relationship because of vessel wall stretching. For these reasons, VOP measurements have until now been strictly limited to the above heart position of the measured region.
While the filling of the veins due to such hydrostatic pressures can be prevented by the application of a sufficient level of external counter pressure to the measurement site (so as to counterbalance the intra-venous pressure), traditional VOP volumetric collecting cups are mechanically incapable of being pressurized to any appreciable degree without being forced off the measurement site. They are therefore unable to provide the necessary degree of pressure to counterbalance the intra-venous pressure, and thus cannot be used to reliably measure BF below heart level. Substitute collecting devices, such as mercury in silastic rings or circumferential strain gauges, provide no actual pressure field to the bulk of the tissues distal to the venous occlusion cuff, so that resting venous distention cannot be counterbalanced by any of these methods. Furthermore, since they do not actually measure volume but rather an index of circumference, they are not capable of being accurately calibrated to provide an index of volume.
Because of these limitations, traditional VOP devices are not able to modify the local venous transmural blood pressure so as to allow BF measurements or venous capacitance at specific pressure levels to be made when the measured body part is below heart level, and therefore, as noted above, VOP measurements have generally been limited to body regions which are located above the subject's heart or are deliberately elevated above heart level.
Further information regarding VOP measurements is available from the patent literature, e.g. U.S. Pat. Nos. 5,447,161 and 6,749,567, the contents of which are incorporated herein by reference. These patents also describe other physiological parameters derived from volumetric blood flow which may also be measured by the VOP method.